For various reasons, it may be desirable to determine the weight of the patient lying on a hospital bed. Hospital beds typically comprise a plurality of load cells which are distributed across the area under a sleep surface and are secured to a patient support frame which is provided under the sleep surface.
Some beds comprise three or four load cells which are located generally at the corners or near the perimeter of a sleep surface of the bed. The load cells are generally provided on a patient support frame which is located directly under the sleep surface. The load cells serve two purposes: determining the weight of the patient by calculating a sum of the weight measured by each load cell, and monitoring patient position on the bed by calculating which proportion of the total weight of the patient is measured by each load cell. Examples of this type of bed are shown in U.S. Pat. Nos. 5,276,432 and 5,802,640.
In this type of arrangement, the load cells are configured to measure loads which are applied in a purely vertical direction on them. However, the patient support frame in most hospital beds comprise a plurality of sections which can be angled relative to each other. In this case, the weight of the patient creates a load which is also angled. Additional “compensation” calculations involving trigonometry may therefore be necessary in order to determine vertical components of the load corresponding to the weight of the patient, which can introduce precision errors in the measured weight.
Furthermore, these systems are costly due to the use of at least three load cells. Their installation is also quite complex because they have to account for mechanical hysteresis in the moving parts of the bed which can affect the precision of the weight measurements. Typically, the patient position system requires a lot less precision from the system than the scale system, but since both systems use the same sensors, the implementation of the patient position monitoring system remains costly.
Other beds include external accessories which are surfaces including a large number of load cells which are placed under the mattress or directly under the patient. An example of this type of bed is shown in U.S. Pat. No. 5,393,935. These accessories are frequently damaged and must be replaced periodically. They must also be cleaned periodically, which further increases the cost of this technology.
To accurately measure weight using load cells, it may also be necessary to reduce lateral forces applied on the load cell, which can cause torsion in the load cells and disturb the weight measurements. In order to reduce these lateral forces, some solutions have been proposed, including rigidifying the frame to reduce deflection of the frame caused by bending and placing the load cells relatively close to the patient. However, these solutions can be costly and complex because they involve redesigning a large portion of the frame.
It has been proposed to mount the sleep surface on a movable frame and to movably connect the movable frame to a fixed frame which sits on the ground with the load cells in order to isolate the purely vertical load created by the weight of the patient. US Patent Publication No. 2015/0157520, for example, uses elastic members to connect the two frames together. However, this connection may still transmit some lateral forces to the load cells. Furthermore, a lateral push on the side of the bed may cause undesirable movement of the sleep surface relative to the fixed frame.
Examples of prior art hospital beds are described in U.S. Pat. Nos. 4,926,951, 5,173,977, 5,859,390, 5,906,016, 6,362,439, 5,276,432, 5,393,935, 4,974,692, 6,924,441, 5,802,640, 6,438,776, 7,253,366, 7,703,158 and 8,921,717, and US Patent Publication No. 2015/0157520.